Method for manufacturing a bevelled stone, particularly for a horological movement

ABSTRACT

A method and device for manufacturing a bevelled stone, particularly for a timepiece are disclosed. A precursor is produced from a mixture of at least one material in powder form with a binder. The method includes pressing the precursor so as to form a green body, using a top die and a bottom die comprising a protruding rib, sintering the green body so as to form a body of the future stone in at least one material, the body including a peripheral face and a bottom face provided with a groove, and machining the body including a substep of planning the peripheral face up to the groove, such that an inner wall of the groove forms at least a flared part of the peripheral face of the stone.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to European Patent Application No.19188535.9 filed on Jul. 26, 2019, the entire disclosure of which ishereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing a bevelled stone,particularly for a horological movement.

The invention also relates to a manufacturing system suitable for usingthe method.

BACKGROUND OF THE INVENTION

In the prior art of horology, ruby or sapphire type stones areparticularly used to form endstones or guiding elements, known asbearings, in timepieces. These endstones and guiding elements areintended to come into contact with pivots in order to render the latterrotatable with minimal friction. Thus, they form, for example, all orpart of a bearing-block of a rotatably fitted shaft. The guidingelements generally comprise a through hole in order to insert the pivotshaft therein.

In principle, synthetic stones are used in horological movements. Inparticular, the Verneuil type method for manufacturing monocrystallinestones is known. Polycrystalline type stones also exist, which aremanufactured by pressing a precursor with a view to obtaining a greenbody of the future stone using a pressing tool. The stones are thenmachined to obtain a finished shape to the desired dimensions.

In particular, regarding the polycrystalline stone guiding elements, thepressing tool is for example provided with a wire contributing to theconstruction of a hole blank. Monocrystalline type stones are firstlaser-bored to obtain the hole blank. The final size of the hole issubsequently obtained by means of machining.

The stones are generally sized to be positioned in the bed of abearing-block assembly. Shock-absorbing bearing-blocks particularlyexist, which are suitable for absorbing shocks.

The configuration of a conventional shock-absorbing bearing-block 1 isrepresented by FIG. 1 . An olive domed stone 2 forming an axial guidingelement for the pivot, which is also commonly known as a bearing, isdriven into a bearing-block support commonly known as a setting 3,whereon an endstone 4 is fitted. The setting 3 is held pressing againstthe bottom of a bearing-block 5 by elastic means, generally ashock-absorbing spring 6, arranged to exert an axial force on the toppart of the endstone 4. Such a shock-absorbing bearing-block makes itpossible to absorb shocks along the longitudinal axis of the pivot, theassembly formed of the setting, the domed stone, and the endstone, beingsuitable for moving thanks to the shock-absorbing spring 6.

The setting 3 further includes a flared outer wall arranged in line witha flared inner wall disposed at the periphery of the bottom of thebearing-block 5. Alternative embodiments also exist whereby the settingincludes an outer wall having a surface of convex shape, i.e. domed.Thus, the shock-absorbing bearing-block makes it possible to absorbradial shocks, thanks to the flared walls of the setting and of thebottom of the bearing-block 5. Indeed, the assembly can move radiallyand axially simultaneously.

However, it is sought to have a single stone capable of forming both theguiding element and the setting to simplify the arrangement of theelements and enhance the shock resistance. For this, it is necessary toobtain a stone with an at least partially flared peripheral wall.However, current machining methods are complex to implement in order toachieve this.

SUMMARY OF THE INVENTION

The aim of the present invention is that of remedying in full or in partthe drawbacks cited above by providing a method for manufacturing astone provided with an at least partially flared peripheral face, inorder to be capable of being inserted in a shock-absorbing bearing-blockas described above. In this context, such a manufacturing method isrepeatable, without causing any damage of the stone or indeed of thesystem for manufacturing this stone.

For this purpose, the invention relates to a method for manufacturing abevelled stone, particularly for a timepiece. The method is remarkablein that it includes the following steps:

-   -   producing a precursor from a mixture of at least one material in        powder form with a binder;    -   pressing the precursor, using a top die and a bottom die        comprising a protruding rib so as to form a green body,    -   sintering said green body so as to form a body of the future        stone in said at least one material, the body comprising a        peripheral face and a bottom face provided with a groove, and    -   machining the body including a substep of planing the peripheral        face up to the groove such that an inner wall of the groove        forms at least a flared part of the peripheral face of the        stone,

Thus, such a method makes it possible to readily manufacture a bevelledstone provided with an at least partially flared peripheral face, thestone being preferably of polycrystal type. Such a stone can thus beinserted into a bearing-block having a shape corresponding to the flaredface, particularly for absorbing lateral shocks.

According to a particular embodiment of the invention, the machiningcomprises a substep of recessing a recess in the top face of the body.

According to a particular embodiment of the invention, the machiningcomprises a substep of cutting the top face of the body, in order toobtain a top face giving the stone a predetermined thickness.

According to a particular embodiment of the invention, the pressingcomprises the recessing of a hole blank by means of a punch of thebottom die.

According to a particular embodiment of the invention, the groove isembodied to be circular and/or centred on the bottom face of the stone.

According to a particular embodiment of the invention, the protrudingrib and the groove have a substantially triangular cross-section, one ofthe sides whereof will form the flared part of the peripheral face ofthe stone, after planing.

According to a particular embodiment of the invention, the groovecomprises an inner face and an outer face, the outer face being removedduring planing.

According to a particular embodiment of the invention, the inner face isretained during planing so as to form the flared part of the peripheralface.

According to a particular embodiment of the invention, the pressing stepis carried out by moving the top and bottom dies closer together in acasing.

According to a particular embodiment of the invention, the sinteringstep includes pyrolysis.

According to a particular embodiment of the invention, the methodincludes a step of finishing the stone, for example lapping and/orbrushing and/or polishing.

According to a particular embodiment of the invention, the material inpowder form is ceramic-based and includes at least a metal oxide, ametal nitride or a metal carbide.

According to a particular embodiment of the invention, the ceramic-basedmaterial in powder form includes aluminium oxide.

According to a particular embodiment of the invention, the ceramic-basedmaterial in powder form further includes chromium oxide.

The invention also relates to a system for manufacturing a stone usingthe method according to the invention. The system is remarkable in thatit comprises:

a device for producing a precursor from a mixture of at least onematerial in powder form with a binder;

a device for pressing the precursor comprising top and bottom diesarranged movable in a casing helping form a green body of the futurestone, the bottom die comprising a protruding rib;

a device for sintering said green body; and

a device for machining a body of the future stone.

According to a particular embodiment of the invention, the protrudingrib is substantially circular and/or centred on the bottom die.

According to a particular embodiment of the invention, the protrudingrib has a substantially triangular cross-section.

BRIEF DESCRIPTION OF THE DRAWINGS

Further specificities and advantages will emerge clearly from thedescription given hereinafter, which is by way of indication and in noway limiting, with reference to the appended drawings, wherein:

FIG. 1 represents schematically a transverse cross-section of ashock-absorbing bearing-block according to a known embodiment from theprior art;

FIG. 2 is a representation of a stone obtained with the method accordingto the invention;

FIG. 3 is a logic diagram relating to the method for manufacturing astone according to the invention;

FIG. 4 is a schematic representation of a part of the green bodyobtained after the pressing step of the method according to theinvention;

FIG. 5 is a schematic representation of a part of the stone after afirst machining substep of the method according to the invention;

FIG. 6 is a schematic representation of a part of the stone after asecond machining substep of the method according to the invention;

FIG. 7 is a schematic representation of a part of the stone after athird machining substep of the method according to the invention;

FIG. 8 is a schematic representation of a system for manufacturing astone according to the invention;

FIG. 9 is a schematic view of a pressing device of the system accordingto the invention; and

FIG. 10 is a schematic view of a bottom die of the pressing device ofthe system according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As explained above, the invention relates to a method for manufacturinga stone suitable for forming a guiding element of a timepiece. The stoneis for example intended to come into contact with a pivot in order torender the latter rotatable with minimal friction. It is thereforeunderstood that the present invention particularly makes it possible toproduce a stone suitable for forming all or part of a bearing-block of arotatably fitted shaft.

FIG. 2 is an example of a bevelled stone 8 obtained thanks to the methodaccording to the invention. Advantageously, the stone 8 is traversed bya hole 9 intended to receive a pivot, also known as a trunnion. Thestone 8 includes a top face 11 and a bottom face 12 of which onecomprises a functional element, herein a cone 13, communicating with thethrough hole 9. In other words, the hole 9 communicates with the topface 6 and also with a substantially conical hollow defined in thebottom face 9. This hollow then forms an engagement cone 13 of the boredstone 8.

In the description, reference is made to the terms top face and bottomface with reference to the figures, particularly to distinguish them.Nevertheless, the top and bottom faces may be inverted.

It is also noted that an inner wall of the body of this stone defined atthe level of the hole 9 includes a rounded zone 14 intended to minimisethe contact with the pivot but also to facilitate any lubrication. Itshould be noted that minimising the contact with the pivot particularlymakes it possible to reduce friction with the pivot.

The bevelled stone 8 further has a partially flared peripheral face 15,and connecting the bottom face 12 of lesser surface area to the top face11 of greater surface area. The flared part 16 is intended to be incontact with an inner wall of a bearing-block so as to absorb thelateral shocks, the stone being able to slide on the wall of the blockthanks to the flared wall, as in the example in FIG. 1 .

The stone 8 is, preferably, formed from a polycrystalline type mineralbody, the body comprising for example al2O3Cr type polyruby or ZrO2 typezirconia.

The manufacturing method 20 of such a stone, which is represented inFIG. 3 , includes six steps.

A first step 21 consists of producing a precursor from a mixture of atleast one material in powder form with a binder. This material may benon-restrictively and non-exhaustively ceramic. This step is intended toform a precursor from a ceramic-based powder set in the binder.

In this context, the ceramic-based powder may include at least a metaloxide, a metal nitride or a metal carbide. By way of example, theceramic-based powder may include aluminium oxide in order to formsynthetic sapphire or a mixture of aluminium oxide and chromium oxide inorder to form synthetic ruby, or indeed zirconium oxide. Furthermore,the binder may be of varied types such as, for example, polymer types ororganic types.

The method then includes a second step 22 of pressing the precursorusing a top die and a bottom die of a pressing device, represented inFIG. 9 , in order to form the green body of the future stone.

The method includes a third step 23 of sintering the green body in orderto form a body 30 seen in FIG. 4 in the material which may be, asmentioned above, ceramic. In other words, this step 23 is intended tosinter the green body in order to form a ceramic body 30 of the futurebored stone. Preferentially according to the invention, the sinteringstep 23 may include pyrolysis.

According to the invention, the body 30 comprises a peripheral face 37and a bottom face 32 provided with a groove 40. FIG. 4 shows the body 30of the stone after the sintering step, and the superimposed final stone8 obtained at the end of the manufacturing method following thedifferent steps of machining the body. The body comprises a top face 31and a bottom face 32. The body 30 further comprises a hole blank 34provided with top and bottom parts 35, 36 which are of different shapes.Indeed, the bottom part 35, which forms the blank of the functionalelement has a conical shape and the top part 36 which comprises theblank of the hole 34 has a cylindrical shape. Such a hole 34 alsocomprises a first opening 37 defined in the body 30 and opening into thebottom face 32 of this body 30.

It should be noted that such a blank particularly makes it possible toform the engagement cone 13 of the stone 8 for easier fitting of thepivot particularly when fitting same blind into the bored stone formingin this example a guiding element. It is therefore understood that theshape of the through hole 9 is provided by the shape of a punch of thebottom die of the pressing device. Thus, such a step 22 of pressing thedevice in FIG. 3 , is intended to compress, using the top die and thebottom die, the precursor in order to form said green body of the futurestone 8.

According to the invention, the body 30 in FIG. 4 comprises a groove 40on the bottom face 32 thereof. The groove 40 describes, preferably, acircular path centred around the hole blank 34. The groove 40 has,preferably, a cross-section of substantially triangular shape.Nevertheless, other shapes are possible, and may for example be slightlyconvex or concave. As shown in the figure, the diameter of the circularpath of the groove 40 determines the width of the final stone.

The groove 40 is circular and centred on the bottom face of the stone.The groove has a substantially triangular cross-section. The groovecomprises an inner face 41 and an outer face 42 forming the two edges ofthe triangular cross-section thereof, the third edge 43 being open ontothe bottom face of the body. The inner face 41 will form the flared partof the peripheral face, whereas the outer face 42 will be subsequentlyremoved. The periphery of the final stone is defined by the groove 40.The groove 40 is formed by the bottom die of the pressing device,represented in FIGS. 9 and 10 , the bottom die including a negativeshape of the groove 40, such as an annular protruding rib.

In FIG. 3 , the method 20 comprises a fourth step 24 of machining thebody 30 of the future stone 8 in FIG. 4 . The fourth step 24 includes afirst substep of recessing a recess 45 in the top face 31 of the body30. The recess 45 is, preferably, substantially hemispherical, as can beseen in FIG. 5 . During this substep, an opening 46 is produced in theblank of the hole 34 suitable for connecting the cone of the bottom part35 to the recess 45. The blank of the hole 34 thus comprises a secondopening 46 defined in the body and opening into the top face 31 of thisbody 30.

The machining step 24 also comprises a second substep of turning forplaning the peripheral wall 37 of the body 30. As shown in FIG. 6 ,material is removed, preferably up to the apex 47 of the groove so as toobtain a peripheral wall 15 that is at least partially flared 16. Thus,the outer face 41 is removed during planing. On the other hand, theinner face 42 is retained so as to form the flared part 16 of theperipheral face 15 of the future stone. The flared part 16 extends fromthe bottom face 12 of the stone 8. A further part 48 of the peripheralwall 15 is substantially straight and connects the flared part 16 of thetop face 31, preferably from the apex 47 of the groove 40. Thissubstantially straight part 48 was formed during the planing of the body30. The dimensions of the bottom face 12 of the stone 8 are determinedby the groove 40, in particular by the junction 49 of the inner face 42to the bottom face 32 of the body 30.

According to an alternative embodiment not shown, the peripheral facemay be entirely flared. In this case, the flared part extends from thebottom face of lesser diameter, to the top face of greater diameter.

The machining step 24 further comprises a third substep of cutting thetop face 31 of the body 30, in order to obtain a top face 11 giving thestone 8 a predetermined thickness, such as that represented in FIG. 7 .The thickness of the stone 8 is chosen according to the configuration ofthe bearing-block.

The machining step 24 is preferentially carried out using laser typedestructive radiation in order to obtain very precise etching. However,this step 24 may be obtained using other types of processes such as, forexample, mechanical recessing such as mechanical boring or high-pressurewater blanking.

Finally, a fifth finishing step 25 makes it possible to give the stone 8a surface roughness compatible with the use thereof. Such a finishingstep 25 may thus include lapping and/or brushing and/or polishingenabling adjustment of the final dimensions and/or removal of edgesand/or local modification of roughness. It is sought for example toobtain a surface roughness Ra=0.025 μm. Such a finishing step 25 maythus include lapping and/or brushing and/or polishing enablingadjustment of the final dimensions and/or removal of edges and/or localmodification of roughness.

With reference to FIG. 8 , the invention also relates to a system 50 formanufacturing the stone. This system 50 comprises the followingdifferent devices:

-   -   a device for producing 51 a precursor from a mixture of at least        one material in powder form with a binder;    -   a device for pressing 52 the precursor comprising top and bottom        dies 57, 58 arranged movable in a casing 59 helping form a green        body of the future stone 8;    -   a device for sintering 53 said green body; and    -   a device for machining 54 the body 30 of the future stone 8        obtained from sintering the green body.

It should be noted that at least two of these devices 51 to 54 can forma single entity of the system 50 together. Such a system 50 is suitablefor carrying out a method for manufacturing the stone 8 represented inFIG. 2 via the steps in FIGS. 4 to 7 .

In the pressing device 52 seen in FIGS. 9 and 10 , each die 57, 58 isfastened to a dual-acting press. According to the invention, one of thedies 57, 58 (or both) is moved closer to the other along the directionsA in the casing 59 of this pressing device 52 in order to form not onlythe top and bottom faces 31, 32 of a body 30 of the future stone 8, butalso the peripheral face 37 of this body 30. In this pressing device 52,the dies 57, 58 are substantially planar and the bottom die 58 includesa punch 56 intended to form a one-eyed cavity 34 in the body 30. Thispunch 56 comprises a main part having a conical shape and an essentiallycylindrical distal part which is provided with a point. The main partand the distal part are intended to form respectively the bottom part 35and the top part 36 of the one-eyed cavity 34.

According to the invention, the bottom die 58 further includes aprotruding rib 60 configured to form the groove 40 on the bottom face 32of the body 30. The protruding rib 60 is, preferably, circular andcentred on the bottom die 58. The protruding rib 60 has dimensions and ashape corresponding to those of the groove 40, because it forms thegroove 40 in the bottom face of the body 30. Thus, the protruding rib 60also has, preferably, a substantially triangular cross-section. Thebottom die 58 comprises a face 62 delimited by the protruding rib 60 andcorresponding to the dimensions of the bottom face 12 of the stone 8.During pressing, the bottom die 58 equipped with the punch 56 and theprotruding rib 60, imprints the bottom face 32 of the body 30 with theshapes thereof. Thanks to this system 50, the green body sought isobtained, which is subsequently sintered to form the body 30, which willmake it possible to arrive at the stone 8 after machining.

Obviously, the present invention is not restricted to the exampleillustrated but is suitable for various alternative embodiments andmodifications which will be obvious to those skilled in the art. Inparticular, other types of functional elements formed by othergeometries of protruding ribs and/or dies 57, 58 may be envisagedadvantageously according to the invention.

The invention claimed is:
 1. A method for manufacturing a bevelled stonefor a timepiece, comprising: producing a precursor from a mixture of atleast one material in powder form with a binder; pressing the precursorso as to form a green body, using a top die and a bottom die, the bottomdie comprising a protruding rib without the top die comprising aprotruding rib; sintering said green body so as to form a body of thefuture stone in said at least one material, the body comprising aperipheral face and a bottom face provided with a groove; and machiningthe body including planing the peripheral face up to the groove, suchthat an inner wall of the groove forms at least a flared part of theperipheral face of the stone.
 2. The method according to claim 1,wherein the machining comprises recessing a recess in a top face of thebody.
 3. The method according to claim 1, wherein the machiningcomprises cutting a top face of the body, in order to obtain a top facegiving the stone a predetermined thickness.
 4. The method according toclaim 1, wherein the pressing comprises a recessing of a hole blank witha punch of the bottom die.
 5. The method according to claim 1, whereinthe groove is embodied to be circular and/or centered on the bottom faceof the body.
 6. The method according to claim 1, wherein the protrudingrib and the groove have a substantially triangular cross-section, one ofthe sides whereof will form the flared part of the peripheral face ofthe stone, after planing.
 7. The method according to claim 1, whereinthe groove comprises an inner face and an outer face, the outer facebeing removed during planing.
 8. The method according to claim 7,wherein the inner face is retained during planing so as to form theflared part of the peripheral face.
 9. The method according to claim 1,wherein the pressing is carried out by moving the top and bottom diescloser together in a casing.
 10. The method according to claim 1,wherein the sintering includes pyrolysis.
 11. The method according toclaim 1, further comprising finishing the stone, including lappingand/or brushing and/or polishing.
 12. The method according to claim 1,wherein the material in powder form is ceramic-based and includes atleast a metal oxide, a metal nitride or a metal carbide.
 13. The methodaccording to claim 12, wherein the ceramic-based material in powder formincludes aluminium oxide.
 14. The method according to claim 13, whereinthe ceramic-based material in powder form further includes chromiumoxide.